1. Field of the Invention
This invention relates to a reflecting liquid crystal display device for displaying with utilizing the external light such as natural light, and more particularly relates to a technology for improving the field angle of a reflection type liquid crystal display device.
2. Description of Related Art
A liquid crystal display device which utilizes liquid crystal as electro-optic material is featured in a flat-panel shape, light weight and thin configuration, and utilizes low power consumption. The liquid crystal display device has been developed very actively for use as a display of various potable apparatus. The liquid crystal is not emissive but forms an image by the mixture of patterned transmission and cutting off of the external light. Such passive liquid crystal display device is classified into transmission type and reflection type. A transmission type liquid crystal display device comprises a panel on which liquid crystal is held disposed between a pair of transparent substrates and a back light disposed on the back side of the panel, and an image is viewed from the front of the panel. A back light is essential for a transmission type liquid crystal display device, for example, a cold-cathode tube is used as the back light. A back light consumes the most of power supplied to the whole display, and is not suitable for the display of a portable apparatus. On the other hand, a reflection type liquid crystal display has a reflection plate on the back side of a panel, an external light such as natural light enters and an image of the reflected light is viewed from the front. Because a back light is not used differently from the transmission type, a reflection type display consumes relatively low power and is suitable as the display for portable apparatus.
In a liquid crystal display device, liquid crystal is birefringent, which is oriented controllably depending on various operation modes suitably. Due to this mechanism, a liquid crystal display device is field angle dependent, and is disadvantageous in that the contrast changes depending on the field angle of an viewer for viewing the screen. As shown in FIG. 5, the field angle of a viewer is represented by a declination (magnetic declination) .phi. and inclination (polar angle) .theta.. In FIG. 5, the declination .phi. is represented by angular deviation in the anti-clockwise direction with respect to the orientation direction of liquid crystal of the panel. The inclination .theta. is represented by inclination with respect to the normal line of the panel 0. Heretofore, a so-called twist nematic (TN) mode liquid crystal panel has been used for reflection type liquid crystal display devices. FIG. 6 shows a field angle characteristics of a reflection type TN mode liquid crystal panel. FIG. 6 is an iso-contrast curve formed by plotting points where the contrast is equal each other on a graph having the declination .phi. in the circumferential direction and the inclination .theta. in the radial direction. As shown in this graph, a reflection type TN mode liquid crystal display is remarkably field angle dependent, and in this example, the display is designed so that the contrast is maximized at .phi.=180 degrees from which a viewer views the screen. In a reflection type TN mode liquid crystal panel, the high contrast region appears in the rising direction of liquid crystal molecules by applying a voltage in the average molecule major axis direction of liquid crystal molecules and the low contrast region appears in the reverse direction. Usually, a panel is designed so that the high contrast region is faced to a viewer. This design method is effective only when the reflection plate is perfectly diffusive. In detail, the above-mentioned design is effective for the case in which the reflection plate is relatively diffusive and incident light comes widely from various direction for being viewed by a viewer. On the other hand, in the case that a reflection plate is not perfectly diffusive and directional, the incident light entered from the direction which is symmetrical to the viewer direction to the panel is viewed by a viewer mainly because a significant gain is added to the reflection characteristics, the design shown in FIG. 6 is not effective. As described herein above, it is necessary to give full consideration to the optical characteristics of a reflection plate in addition to the optical characteristics of a liquid crystal panel itself in designing the field angle of a reflection type panel.
The display screen of a reflection type TN mode liquid crystal display is dark and not convenient for use because two deflecting plates are incorporated. The TN-ECB mode display, which utilizes one deflecting plate, is promising as an reflection type liquid crystal display device in view of the display contrast. The TN-ECB mode is a system which utilizes retardation of liquid crystal basically, but the principle of TN mode is taken into the design from the view point of panel design. The reflection type TN-ECB mode is also field angle dependent, however, the field angle characteristics is different from the above-mentioned TN mode. In some cases, the reflection plate incorporated ina refection type TN-ECB mode panel is different from that incorporated in a usual TN mode panel. Heretofore, no research and development has been conducted to optimize the field angle of the reflection type TN-ECB mode liquid crystal display device, and the problem has remained unsolved.